title: LED-array photocalorimetry: instrument design and application to photostability of nifedipine.
abstract: A photocalorimeter was designed to analyse quantitatively the photostability of pharmaceuticals. Its application is demonstrated with reference to two solution phase test systems; the photodegradations of 2-nitrobenzaldehyde (2-NB) and nifedipine. Light emitting diode (LED) arrays were used to illuminate the sample and reference channels of the calorimeter. Five LEDs were used to create an array from 360 to 700nm. A power supply system was constructed that zeroed the instrument by supplying a preset voltage to the sample side LEDs and varying that supplied to the reference LEDs until a zero calorimetric signal was obtained. The photodegradation of 2-NB was zero-order and varied in proportion to the input voltage supplied to the LED array. Analysis of the data (the rate of reaction was determined to be equal 1.04x10(-6)moldm(-3)s(-1) by pH titration) determined a reaction enthalpy of 5.0+/-0.6kJmol(-1). In the case of nifedipine, the LEDs in the array were operated individually in order to determine the causative wavelength of degradation. This was found to be 360nm, in agreement with the literature.
authors.affiliation: The School of Pharmacy, University of London, 29-39 Brunswick Square, London WC1N 1AX, UK.
authors.forename: Meena
authors.initials: M
authors.lastname: Dhuna
authors.forename: Anthony E
authors.initials: AE
authors.lastname: Beezer
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Connor
authors.forename: David
authors.initials: D
authors.lastname: Clapham
authors.forename: Chris
authors.initials: C
authors.lastname: Courtice
authors.forename: John
authors.initials: J
authors.lastname: Frost
authors.forename: Simon
authors.initials: S
authors.lastname: Gaisford
mesh_terms: D002151:Calorimetry / Q000295:instrumentation*; D004355:Drug Stability; D004581:Electronics; D004867:Equipment Design / Q000295:instrumentation*; D006863:Hydrogen-Ion Concentration; D008027:Light; D009543:Nifedipine / Q000737:chemistry / Q000528:radiation effects; D010777:Photochemistry / Q000295:instrumentation*; D013816:Thermodynamics
chemical_list: D009543:Nifedipine